U.S. patent application number 09/955662 was filed with the patent office on 2002-03-21 for method and apparatus for maximizing the use of available capacity in a communication system.
Invention is credited to Lundby, Stein A., Tiedemann, Edward G. JR..
Application Number | 20020034170 09/955662 |
Document ID | / |
Family ID | 23006067 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020034170 |
Kind Code |
A1 |
Tiedemann, Edward G. JR. ;
et al. |
March 21, 2002 |
Method and apparatus for maximizing the use of available capacity
in a communication system
Abstract
A method and apparatus for maximizing the use of available
capacity in a communication system. The forward link in the mobile
radio system includes a plurality of traffic streams sent on at
least one channel from the base station to the mobile stations. A
first output power level associated with simultaneously
transmitting a first set of traffic streams from the base station
to the mobile stations on the forward link is initially determined.
Next, the first output power level is compared to the maximum power
ceiling. In response to the comparing step, at least one time frame
in the forward link having available capacity for transmitting a
portion of at least one further traffic stream is identified. The
first set of traffic streams and the portion of the at least one
further traffic stream are then transmitted simultaneously during
the at least one frame on the forward link.
Inventors: |
Tiedemann, Edward G. JR.;
(San Diego, CA) ; Lundby, Stein A.; (San Diego,
CA) |
Correspondence
Address: |
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
23006067 |
Appl. No.: |
09/955662 |
Filed: |
September 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09955662 |
Sep 19, 2001 |
|
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09264435 |
Mar 8, 1999 |
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6317435 |
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Current U.S.
Class: |
370/335 ;
370/342; 370/414; 370/441; 455/522; 455/67.11 |
Current CPC
Class: |
H04B 7/2631 20130101;
H04W 24/00 20130101; H04W 52/50 20130101; H04W 52/48 20130101 |
Class at
Publication: |
370/335 ;
370/441; 370/342; 370/414; 455/522; 455/67.1 |
International
Class: |
H04B 007/216 |
Claims
What is claimed is:
1. A method for transmitting information from a base station to
mobile stations in a communication system, the method comprising
the steps of: (A) identifying at least one portion of a time frame
within the forward link, the identified portion of the frame having
available capacity for transmitting at least a portion of at least
one previously unscheduled traffic stream in addition to any
traffic streams previously scheduled to be transmitted over the
forward link; and (B) simultaneously transmitting the previously
scheduled traffic streams and the portion of the previously
unscheduled traffic stream during the identified portion of the
frame.
2. A method for transmitting information from a base station to
mobile stations in a communication system, the method comprising
the steps of: (A) identifying at least one portion of a time frame
within the forward link, the identified portion of the frame having
available capacity for transmitting at least a portion of at least
one previously unscheduled traffic stream in addition to any
traffic streams previously scheduled to be transmitted over the
forward link; and (B) simultaneously transmitting the previously
scheduled traffic streams and the portion of the previously
unscheduled traffic stream during the identified portion of the
frame, wherein a sum of the power allocated to the scheduled and
unscheduled traffic streams is no greater than a maximum power
ceiling.
3. The method of claim 2, wherein the sum is substantially equal to
the maximum power ceiling and the sum is maintained at a constant
level over a plurality of time frames by repeating the steps of
claim 1.
4. The method of claim 2, wherein at least a portion of one frame
in the previously unscheduled set of traffic streams is
intentionally transmitted at a first symbol energy that is
insufficient for correct demodulation by an intended receiving
station.
5. The method of claim 4, further comprising the step of
retransmitting on the forward link at least one portion of the
information previously transmitted at the first symbol energy
amount, wherein the retransmitted portion is retransmitted with a
symbol energy that is insufficient by itself for correct
demodulation by the intended receiving station.
6. The method of claim 5, repeating retransmission of the
retransmitted portion until the sum of the symbol energy received
is great enough to permit correct demodulation of the retransmitted
portion by the intended receiving station.
7. The method of claim 6, wherein the previously scheduled traffic
streams includes at least one constant bit rate traffic stream and
at least one variable bit rate traffic stream.
8. The method of claim 7, wherein frames in the constant bit rate
traffic stream and frames in the previously unscheduled traffic
streams are offset in time with respect to each other.
9. The method of claim 8, wherein frames in the previously
unscheduled traffic streams include messages that have different
lengths.
10. The method of claim 2, wherein a traffic stream from the
previously unscheduled streams has a different frame length than a
traffic stream from the previously scheduled steams.
11. The method of claim 10, wherein the further traffic stream is
transmitted discontinuously.
12. The method of claim 11, wherein the previously unscheduled
traffic stream has a lower priority than the previously scheduled
traffic streams.
13. The method of claim 2, wherein frames in at least one of the
previously scheduled traffic streams and frames in the at least one
of the previously unscheduled traffic stream are offset in time
with respect to each other.
14. The method of claim 13, wherein frames in at least one of the
previously scheduled traffic streams and frames in the at least one
of the previously unscheduled traffic stream have different
lengths.
15. The method of claim 2, wherein the communication system uses
code division multiple access (CDMA) modulation.
16. In a radio communication system having a base station and a
plurality of mobile stations, wherein a forward link that includes
a plurality of traffic streams is sent on at least one channel from
the base station to the mobile stations, and the forward link is
subject to a maximum power ceiling, an apparatus for transmitting
information from the base station to the mobile stations,
comprising: (A) a base station controller that determines an output
power level associated with simultaneously transmitting a first set
of one or more traffic streams from the base station to the mobile
stations on the forward link, compares the output power level with
the maximum power ceiling, and identifies at least one time frame
in the forward link having available capacity for transmitting a
portion of a second set of one or more traffic stream; and (B) a
base station transmitter that simultaneously transmits the first
set of one or more traffic streams and the portion of the second
set of one or more traffic stream during the at least one frame on
the forward link.
17. In a radio communication system having a base station and a
plurality of mobile stations, wherein a forward link that includes
a plurality of traffic streams is sent on at least one channel from
the base station to the mobile stations, and the forward link is
subject to a maximum power ceiling, an apparatus for transmitting
information from the base station to the mobile stations,
comprising: (A) means for determining an output power level
associated with simultaneously transmitting a first set of one or
more traffic streams from the base station to the mobile stations
on the forward link; (B) means for comparing the output power level
with the maximum power ceiling; (C) means for identifying at least
one time frame in the forward link having available capacity for
transmitting a portion of a second set of one or more traffic
stream; and (D) means for simultaneously transmitting the first set
of one or more traffic streams and the portion of the second set of
one or more traffic stream during the at least one frame on the
forward link.
Description
CROSS-REFERENCE
[0001] The present Application for Patent is a co-pending
application of U.S. patent application Ser. No. 09/264,435, filed
Mar. 8, 1999, assigned to the assignee hereof and hereby expressly
incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to the field of communication systems
and, in particular, to maximize the use of available capacity in a
communication system where signals associated with multiple users
may be simultaneously transmitted on a common channel.
[0004] 2. Description of the Prior Art
[0005] Telecommunications traffic can be divided into a number of
classes. One classification scheme divides the traffic based upon
the rate at which the traffic is transmitted and the priority of
the traffic. In accordance with this classification scheme, traffic
is classified as constant bit rate (CBR) traffic, variable bit rate
(VBR) traffic, or available bit rate (ABR) traffic. (CBR) traffic
is afforded a fixed bit rate regardless of the requirements of the
data that is to be transmitted. This is the most expensive type of
service available. VBR traffic allows a user to decide the rate at
which the traffic is sent for each communication. ABR traffic is
the lowest priority traffic. ABR traffic is transmitted at whatever
rate is available. Accordingly, ABR service is relatively
inexpensive.
[0006] One example of traffic that is best sent using CBR service
is conventional fixed rate circuit switched traffic. Examples of
signals having the variable demands suitable for VBR service are
speech and Internet video services. Both CBR and VBR traffic are
usually real time with a relatively high quality of service
requirement. The quality of service is an indication of the
reliability that data will be successfully received as well as the
delay involved in the reception. ABR traffic has a lower priority
and does not provide high probability that the traffic will be
delivered within a short time interval. Traffic suitable for ABR
service includes file transfers and electronic mail transfers. If
loading is not high, and delay is therefore not high, most world
wide web transmissions use ABR service.
[0007] The forward link capability of a cellular communication
system (i.e., the number of users and the bit rate of each user) is
in part controlled by the capabilities of the power amplifier used
to amplify the signals transmitted from the base stations of the
system. For example, in a code division multiple access (CDMA)
communication system, each of the traffic streams transmitted is
assigned to a code channel. Details of an exemplary CDMA system can
be found in U.S. Pat. No. 4,901,307 entitled "Spread Spectrum
Multiple Access Communication System Using Satellite Or Terrestrial
Repeaters", which is assigned to the assignee of the present
invention and incorporated herein in its entirety by reference.
Each channel in a CDMA system is modulated over a frequency band
(which is the same for each code channel) and combined to form a
CDMA channel. The amount of power required in each code channel
depends upon the bit rate of traffic transmitted over that code
channel, the gains of the antennas at the receiving station (such
as a mobile station) and a transmitting station (such as a base
station), the path loss (i.e., the amount of attenuation of the
signal) between the base station and the remote station to which
the information is sent, the noise level at the mobile station, and
the performance of the modulation scheme used. The noise level at
the mobile station includes thermal noise, noise from other cells
that the mobile station is not receiving, and noise from
non-orthogonal signal components from the cell that the mobile
station is receiving. The CDMA channel is amplified by the power
amplifier within the base station. The base station must transmit a
total power sufficient for an intended receiving mobile station to
receive the signals directed to it at the desired error rates. The
base station uses various procedures so that the total amount of
power required by the CDMA channel does not exceed the amount of
power that the power amplifier can provide without undesirable
distortion.
[0008] The forward link capability of a cellular communication
system is also limited by the amount of interference from the
user's own cell (from non-orthogonal components if the waveform is
transmitted orthogonally as in TIA/EIA-95) and by the interference
from signals transmitted by other cells. This provides a limit
irrespective of the amount of power that the base station
transmits. In this situation, increasing the base station's
transmission power above some limits only marginally increases the
capability of the system.
[0009] The maximum output power level of a base station is
determined by a number of design parameters related to the power
amplifier of the base station. Two relevant parameters of the power
amplifier include power dissipation and unwanted emissions.
Unwanted emissions are emissions that are outside the bandwidth of
a transmitted signal. A large portion of the unwanted emissions
occur due to intermodulation within the power amplifier.
Intermodulation is a form of distortion. Intermodulation distortion
increases as the power amplifier is driven closer to the maximum
output of the amplifier. Regulatory bodies, such as the Federal
Communication Commission often limit unwanted emissions. Industry
standards can also set limits on unwanted emissions in order to
avoid interference with the same system or another system.
[0010] In order to maintain unwanted emissions within the required
limits, the output power capability of a power amplifier is
selected to provide a very small probability that the unwanted
emissions will exceed the required limit. When the requested power
exceeds the maximum output power, a base station can limit the
output power in order to maintain the unwanted emissions within the
prescribed limits. However, the demand on the power amplifier is
determined by the number of traffic streams that are transmitting
at the same time. Each transmitted traffic stream can start and end
arbitrarily. Therefore, it is difficult to determine the amount of
power that the base station is required to transmit at any
particular time.
[0011] An important measure in a communication system is the
signal-to-noise ratio. In a digital communication system, the
required signal-to-noise ratio is equal to the product of the bit
rate and the required energy per bit divided by the total noise
spectral density. The error rate of the communication system is
often expressed in terms of the bit error rate or the frame error
rate. The error rate is a decreasing function of the
signal-to-noise ratio. If the received signal-to-noise ratio is too
low, then the probability that an error will occur is very high.
Thus, a communication system attempts to maintain the received
signal-to-noise ratio at or above the required signal-to-noise
ratio for the desired error rate.
[0012] Accordingly, in mobile radio communication systems such as
CDMA systems, where multiple users simultaneously transmit on a
common channel, the number of simultaneous VBR and CBR users
permitted within telecommunication system is usually limited. The
limit is selected to maintain a low probability of exceeding the
maximum output power. When selecting the limits on the number of
users, the variable rate nature of the VBR services and the dynamic
power control on the forward link must be considered.
[0013] While the characteristics set forth above have been
described in connection with the forward link, similar
characteristics also apply to the reverse link.
SUMMARY OF THE INVENTION
[0014] A method for maximizing the use of available capacity in a
communication system (such as a CDMA system) that uses a common
frequency channel for simultaneously transmitting signals
associated with multiple users is disclosed herein. In accordance
with the disclosed method, a forward link in a mobile radio system
supports a plurality of traffic streams associated with multiple
users and is sent on at least one common channel from a
transmitting station (such as a base station) to receiving stations
(such as mobile stations). The forward link is subject to a maximum
power ceiling. A first output power level associated with
simultaneously transmitting a first set of traffic streams from the
base station to the mobile stations on the forward link is
initially determined. Next, the first output power level is
compared to a maximum power ceiling. At least one time frame in the
forward link having "available capacity" for transmitting a portion
of at least one further traffic stream is identified. Having
available capacity, means that the amount of power required to
transmit the forward link is lower than the power level at which
the forward link can be transmitted without undesirable distortion.
The first set of traffic streams and the portion of the at least
one further traffic stream are then transmitted simultaneously
during the at least one frame on the forward link. The further
traffic stream may optionally be transmitted discontinuously on the
forward link and have a lower priority than the first set of
traffic streams. Discontinuous transmission refers to the
transmission over frames that are not adjacent to one another in
time (i.e., frames which do not include the discontinuous stream
are transmitted between frames that do include the discontinuous
stream).
[0015] In accordance with a preferred embodiment, any available
capacity on the forward link is allocated to a second set of
traffic streams in which each member of the second set is
transmitted discontinuously on the forward link by using one or
more frames. In this embodiment, a second output power level is
associated with simultaneously transmitting the group of frames
from the second set of traffic streams on the forward link, and the
sum of the first output power level (i.e., the output power level
associated with transmitting the first set of traffic streams on
the forward link) and the second output power level is no greater
than the maximum power ceiling.
[0016] In a particularly preferred embodiment, the sum of the first
and second output power levels is maintained at a constant level
(preferably equal to the maximum power ceiling) over a plurality of
time frames. When the present invention is implemented in
connection with a fast forward link power control system, the power
allocation determinations necessary to implement the invention are
preferably made in a power manager located at a base station
transceiver. Alternatively, in cases where the system includes a
base station controller that services a plurality of base station
transceivers, the power allocation determinations may be made in a
scheduler located in the base station controller and then sent to
the appropriate base station transceiver.
[0017] In accordance with a further aspect, in cases in which the
available capacity on the forward link is present over a group of
one or more frames and is allocated to a second set of traffic
streams, at least one frame in the second set of traffic streams is
initially transmitted on the forward link with a first symbol
energy that is insufficient for correct demodulation by an intended
receiving mobile station. In this embodiment, at least one frame in
the second set of traffic streams initially transmitted with the
first symbol energy is retransmitted at a later time with a further
symbol energy that may also be insufficient by itself for correct
demodulation by the intended receiving mobile station. The
retransmission of the at least one frame is performed one or more
times until the sum of the symbol energy received is great enough
to permit correct demodulation by the intended receiving mobile
station.
[0018] In cases where a frame is initially transmitted with a first
symbol energy amount that is insufficient for correct demodulation
by an intended receiving mobile station, that mobile station can
determine that the received frame has been received incorrectly and
inform the base station by use of a predetermined protocol. The
protocol can be either a positive or negative acknowledgement
protocol. In other words, the mobile station can either send an
acknowledgement when it is able to correctly demodulate the
information or, alternatively, the mobile station can send a
negative acknowledgement each time it is unable to correctly
demodulate the information. Since the base station can estimate the
symbol energy of the information received at the mobile station,
the mobile station may, but need not, send energy information back
to the base station when either protocol is employed. Thus, the
explicit transmission of additional energy information from the
mobile station to the base station in order to select the power
level for retransmission of the frame to the mobile station is
optional in the present invention.
[0019] In accordance with a still further aspect, the first set of
traffic streams includes at least one constant bit rate traffic
stream and at least one variable bit rate traffic stream, and
frames in the constant bit rate traffic stream and frames in the
second set of traffic streams are offset in time with respect to
each other. The group of frames in the second set of traffic
streams may optionally include messages that have different
lengths. In addition, each of the traffic streams may have a
different frame length.
[0020] The aspect of the invention that initially transmits traffic
information from a base station with a symbol energy that is
insufficient for correct demodulation at an intended receiving
mobile station, and then later retransmits the same traffic
information from the base station with additional symbol energy
that is also by itself insufficient for correct demodulation at the
intended receiving mobile station, may be applied generally in
forward or reverse link transmissions in order to achieve time
diversity. In other words, this aspect of the invention may be used
to transmit any traffic stream and not simply one of the specific
traffic streams mentioned in the above embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The features, objects, and advantages of the present
invention will become more apparent form the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify corresponding elements
throughout and wherein:
[0022] FIG. 1 shows a graphical representation of the traffic in
the forward link of a cellular communication system for a period
covering a plurality of time frames having available capacity;
[0023] FIG. 2 shows a graphical representation of the traffic in
the forward link of a cellular communication system for a period
covering a plurality of time frames wherein all available capacity
in the forward link has been allocated to ABR traffic;
[0024] FIG. 3 shows a graphical representation of the traffic in
the forward link of a cellular communication system for a period
covering a plurality of time frames wherein time offsets are
applied to transmission signals;
[0025] FIG. 4 shows a graphical representation of the traffic in
the forward link of a cellular communication system for a period
covering a plurality of time frames wherein a predetermined
scheduling policy is applied;
[0026] FIG. 5 shows a scheduling time line of an acknowledgment
protocol between a base station and a mobile station of a
communication system suitable for implementation in the system of
the present invention;
[0027] FIG. 6 shows a scheduling time line of a negative
acknowledgment protocol between a base station and a mobile station
of a communication system suitable for implementation in the system
of the present invention;
[0028] FIG. 7 shows a scheduling time line of a negative
acknowledgment protocol between a base station and a mobile station
of a communication system suitable for implementation in the system
of the present invention;
[0029] FIG. 8 is a block diagram showing a base station controller
that includes a scheduler for allocating forward link power among
different traffic streams in accordance with the present
invention.
[0030] FIG. 9 is a block diagram showing two base station
transceivers that each include a power manager for allocating
forward link power among different traffic streams in accordance
with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0031] FIG. 1 is a graphical representation 10 of the traffic in
the forward link of a cellular communication system. The graphical
representation 10 covers a time period that includes the time
frames 18a-f. The time frames 18a-f can be, for example, twenty
milliseconds in duration. The graphical representation 10
illustrates the use of a communication system to transmit forward
link traffic that includes the three constant bit rate CBR traffic
streams 14a-c. All CBR traffic streams 14a-c are transmitted during
all of the time frames 18a-f. Additionally, three variable bit rate
(VBR) traffic streams 14d-f are shown in the graphical
representation 10. The VBR traffic streams 14d-f alternate between
on and off states and have varying transmission rates during each
time frame 18a-f.
[0032] Traffic streams 14a-f are all transmitted simultaneously on
a common channel using for example, CDMA modulation. Within the
forward link set forth by the representation 10, the time frame 18c
is the most heavily loaded because the output power required of the
base station is the greatest during the time frame 18c. More
specifically, the time frame 18c requires more power than the other
time frames 18a-f because of the requirements of the VBR traffic
streams 14d-f. The time frame 18e is the most lightly loaded
because the two traffic streams 14e, 4f require little power during
the time frame 18e due to the relatively low bit rates. Unfilled
areas 22 of the graphical representation 10 indicate unused power
and therefore available capacity within the communication system
set forth.
[0033] FIG. 2 is a graphical representation 20 of the traffic in
the forward link of a cellular communication system during a period
covering the time frames 18a-f. The graphical representation
illustrates the use of the communication system to transmit
traffic. The transmitted traffic includes the three CBR traffic
streams 14a-c and the three VBR traffic streams 14d-f. The traffic
streams 14a-f are transmitted as previously described with respect
to the graphical representation shown in FIG. 1. Additionally, the
graphical representation of FIG. 2 shows ABR traffic streams 20a,
b. It should be noted that the ABR traffic stream 20a has priority
over the ABR traffic stream 20b. ABR traffic streams 20a, b are
transmitted simultaneously on the same channel as traffic streams
14a-f using, for example, CDMA modulation.
[0034] The ABR traffic streams 20a, b use all of the remaining
available base station output power as represented by the unfilled
areas 22 of the graphical representation shown in FIG. 1. In this
example, the base station loads the forward link with CBR and VBR
traffic in every time frame 18a-f. The base station then determines
which time frames 18a-f have additional capacity available for the
transmission of the ABR traffic by comparing the power needed for
transmission of the CBR and VBR during each such frame with the
maximum output power value. The base station then schedules or
transmits the ABR traffic in order to take advantage of the
available transmit power which would otherwise remain unused. The
transmission of the ABR traffic is performed consistent with the
relative priorities of each of the ABR traffic streams. This
scheduling method is possible in the example shown in FIG. 2
because the frame lengths of the CBR, VBR, and ABR traffic are
identical. It should be understood that CBR or VBR streams can be
used to fill in the available transmit power in the same manner as
ABR streams provided that the quality of service requirements for
these streams can be met.
[0035] The base station can apply different policies to determine
how best to schedule or transmit the ABR traffic streams in order
to take advantage of the available forward link transmit power that
would otherwise remain unused. For example, after determining the
power that will be required to transmit each of the various ABR
streams buffered for transmission, the base station may simply
select one or more ABR streams with power requirements that are
likely to be equal to the available capacity. Alternatively, the
base station may split the available capacity equally among all the
ABR streams buffered for transmission. Furthermore, ABR streams may
be transmitted discontinuously. Discontinuous transmission refers
to the transmission over frames that are not adjacent to one
another in time (i.e., frames which do not include the
discontinuous stream are transmitted between frames that do include
the discontinuous stream).
[0036] As explained more fully below, in scheduling the ABR streams
for transmission, the base station may opt to transmit a given ABR
stream at full power (i.e., at the power level that the base
station estimates is required for correct demodulation of the
transmitted information at the mobile station) or, alternatively,
the base station may opt intentionally to transmit ABR traffic
information initially at less than the full power required for
correct demodulation and then, at a later time, retransmit the same
traffic information again at less than full power. The mobile
station receiving the multiple transmissions of the same traffic
information will then combine (or sum) both transmissions on a
symbol-by-symbol basis in a buffer in order to correctly demodulate
the traffic information. In one embodiment, the base station
allocates power among a number of different streams such that none
of the streams are initially transmitted with enough power for
correct demodulation by the intended receiver. By initially
transmitting the traffic information at less than sufficient power
to be correctly demodulated by an intended receiver and then
retransmitting the same information at a later time, the base
station is able to achieve time diversity in connection with the
ABR transmissions. In a fading environment, this lowers the total
required E.sub.b/N.sub.0. Other parameters that the base station
can adjust in connection with allocating the otherwise unused power
are the transmission rate and the code rate of the transmitted
stream.
[0037] One advantage of completely filling the forward link in the
manner described above is that the total power I.sub.or transmitted
by a base station on the forward link is constant. Consistency in
the loading of the forward link can simplify forward power control.
However, it is not necessary to use all of the available capacity
on the forward link. Furthermore, even if all of the available
capacity is used, it is not necessary to fill the remaining power
entirely with ABR traffic stream(s). For example, if there is
sufficient power to allow additional CBR or VBR traffic streams to
be transmitted over the forward link, then in one example, the
available capacity can be used to transmit such a CBR or VBR
traffic stream.
[0038] FIG. 3 is a graphical representation 30 of the traffic in
the forward link of a cellular communication system during a period
covering the time frames 18a-f. The graphical representation 30
illustrates the use of a communication system to transmit traffic
including the three CBR traffic streams 14a-c and the three VBR
traffic streams 14d-f. Traffic streams 14a-c are transmitted as
previously described with respect to the graphical representations
10, 30. However, within the graphical representation 30, the frames
of the VBR traffic streams 14d-f are offset with respect to the
time frames 18a-f. The frame offsets in graphical representation 30
reduce peak processing, (i.e., the amount of information that must
be processed at the same time), peak backhaul usage (the amount of
information that must be communicated to other infrastructure
components, such as base station transceivers (BTSs) and base
station controllers (BSCs)), and delay within a communication
system. Frame offsets of this type are well known.
[0039] In addition, the offsets shown in FIG. 3 cause the total
required transmit power within time frame 18a-f to vary
substantially. In CDMA radio telephone systems operating in
accordance with the TIA/EIA Interim Standard entitled "Mobile
Station-Base Station Compatibility Standard for Dual-Mode Wideband
Spread Spectrum Cellular System", TIA/EIA/IS-95, dated July, 1993,
the contents of which are also incorporated herein by reference
(the IS-95 standard), there are sixteen possible time offsets
within a time frame 18a-f. The transmit power level can therefore
vary up to sixteen times within each frame. When the transmit power
level varies sixteen times there is some statistical averaging of
the load because the number of traffic streams is large.
Nevertheless, there is still substantial variability in the
transmit power level. This can make the allocation of power for the
ABR streams 20a, b very difficult. However, very fast power control
methods are available. The power control methods typically operate
at eight hundred times per second per stream and therefore increase
or decrease the required transmit power per stream every 1.25
milliseconds. A system for fast forward link power control is
disclosed in U.S. patent application No. 08/842,993, entitled
"METHOD AND APPARATUS FOR FORWARD LINK POWER CONTROL," which is
owned by the assignee of the present application and the contents
of which are incorporated herein by reference.
[0040] Frames 18a-f of the graphical representations 10, 20, 30 are
all of the same duration. In the preferred embodiment they are 20
ms in duration. Additionally, frames of different lengths can be
used. For example, frames having a 5 ms duration that are
intermixed with the frames of length 20 ms can be used.
Alternatively, frames having a longer duration, such as 40 ms can
be intermixed with frames of length 20 ms.
[0041] FIG. 4 is a graphical representation 40 of the traffic in
the forward link of a cellular communication system for a period
covering the time frames 18a-f. The graphical representation 40
illustrates a scheduling policy adapted to maintain the base
station output power level at a constant level. As was the case
with the system shown in FIG. 2, in the system shown in FIG. 4 the
base station schedules ABR traffic streams 20a, 20b in order to
take advantage of the available transmit power (i.e., blocks 22
shown in FIG. 3) which would otherwise have remained unused. The
transmit power level of the ABR traffic streams 20a, b can be
dynamically adjusted in order to maintain the output power
constant. Thus, the base station can reduce the power of the ABR
traffic streams 20a, b if it has insufficient available capacity.
The adjustment can be made in the middle of a 20 ms frame. As a
result, the transmit power level of the ABR traffic streams 20a, b
can be lower than required for adequate reception when using the
dynamic adjustment. Similarly, the base station can increase the
power of the ABR traffic streams 20a, 20b if the base station has
available capacity. The various scheduling policies discussed above
in connection with FIG. 2 may also be applied in the context of the
system shown in FIG. 4.
[0042] Turn now to the disclosed method mentioned above, in which
the base station intentionally transmits ABR traffic information
initially with less than sufficient power required for correct
demodulation by an intended receiver. Those skilled in the art will
understand that successful transmission of a bit of information in
a communication system requires a minimum energy per bit/noise
spectral density, E.sub.b/N.sub.0. The probability of a bit error
is a decreasing function of E.sub.b/N.sub.0. A frame consists of a
number of bits. A frame is in error if any of the bits in the frame
is in error. In an uncoded communication system, a high enough
E.sub.b/N.sub.0 is required for every bit in order for the frame
not to be in error. However, in coded and interleaved systems the
requirement does not necessarily apply to each bit. Rather, these
systems typically require a minimum average E.sub.b/N.sub.0. The
average energy level actually required in coded and interleaved
systems can depend upon the duration of the averaging, in
particular the coding and interleaving, and the amount of energy
received at various times.
[0043] Coding and interleaving are typically used to counter the
effects of fading that often occur in transmission channels. In
communication systems compatible with the IS-95 standard, the
coding and interleaving are performed over the duration of a 20 ms
frame. Thus, in systems of this type the total energy received per
frame is an important quantity. Therefore, it is important for
understanding the relevance of the graphical representations herein
and the system and method of the present invention that a more
detailed description of the transmission energy and error rates be
provided.
[0044] The total energy received per frame can be represented as
E.sub.t/N.sub.0. If there are N coded symbols per frame, each with
equal E.sub.s/N.sub.0, then:
E.sub.t=N E.sub.s/N.sub.0
[0045] where E.sub.s is the energy of a symbol.
[0046] Let (E.sub.s/N.sub.0).sub.rki be the received
E.sub.s/N.sub.0 for the ith symbol of the kth frame. Furthermore,
let (E.sub.t/N.sub.0).sub.r- k be the received energy in the kth
frame. Then the energy to spectral noise density received during
the kth frame can be expressed as: 1 ( E t / N 0 ) rk = i = 0 N - 1
( E S / N 0 ) rki
[0047] The probability that the kth frame is correctly received
(i.e., that the kth frame is received with sufficient energy to
permit correct demodulation by an intended receiver) is
proportional to (E.sub.t/N.sub.0).sub.rk. Thus, if
(E.sub.t/N.sub.0).sub.rk exceeds a predetermined value there is a
high probability that the kth frame is received correctly. The
E.sub.sN.sub.0 that is received at the mobile station can be
determined from P.sub.rC/N.sub.0/R, where P.sub.r is the received
power, C is the code rate, and R is the transmission rate.
Alternatively, the E.sub.s/N.sub.0 can be determined by any one of
the many techniques known to those skilled in the art. In the case
of a system such as an IS-95 system, E.sub.s is the energy per
symbol received on a code channel and P.sub.r is the power received
on the code channel.
[0048] When the transmit power of an ABR traffic stream is
permitted to vary, either the bit rate or the received
E.sub.sN.sub.0 must vary. Rapid varying of the transmitted power of
an ABR traffic stream is desired in order to maintain a high base
station output power level. However, it is difficult to reliably
signal the new transmitted rate to the mobile station. For an IS-95
type system the output power level can change every 1.25
milliseconds as previously described. Thus, the received
E.sub.s/N.sub.0 can be made to vary, and, accordingly, the
(E.sub.t/N.sub.0).sub.rk can vary. The base station wastes power if
it transmits at a power level sufficient to make
(E.sub.t/N.sub.0).sub.rk large enough to provide a very small error
probability. Alternatively, if the base station transmits at a
power level that is too low it can cause the error probability in
the frame to be too high.
[0049] A base station can estimate the received
(E.sub.t/N.sub.0).sub.rk at a mobile station based upon the amount
of power transmitted on the code channel. The base station can
perform this estimation by summing the coded symbol energies that
are transmitted on the code channel. Since the total
(E.sub.t/N.sub.0).sub.rk is a good indication of the probability of
correct frame reception, the base station can determine whether it
has transmitted a high enough energy level to have the desired
probability of correct reception. If the transmitted energy level
is not high enough, the base station can increase its transmit
power level during the later parts of the frame in order to
compensate and approach the desired transmitted
(E.sub.t/N.sub.0).sub.k. Likewise, if the base station transmits
more energy than necessary in the early part of the frame, it can
reduce the amount of energy later in the frame and apply the saved
energy to the remaining code channels. The base station is not
required to actually compute (E.sub.t/N.sub.0).sub.rk, the base
station can, instead, compute a normalized transmitted symbol
energy value. The base station can determine the required
normalized total transmitted energy per frame using any method
known to those skilled in the art.
[0050] As described below, the present invention can be used
without the explicit transmission of additional energy information
from the mobile station to the base station. In particular, the
mobile station can determine whether the received frame is received
correctly or not and perform an acknowledgment protocol with the
base station. The protocol can be either a positive or negative
acknowledgement protocol. In other words, the mobile station can
either send an acknowledgement when it is able to correctly
demodulate the information or, alternatively, the mobile station
can send a negative acknowledgement each time it is unable to
correctly demodulate the information. Two exemplary acknowledgment
protocols that may be used in connection with the present invention
are discussed below in connection with FIGS. 5 and 6. If past power
control is being used, the base station can estimate the symbol
energy of the information received at the mobile station. Then the
mobile station may, but need not, send energy information back to
the base station when either protocol is employed. Thus, the
transmission of such energy information from the mobile station
back to the base is optional in the present invention.
[0051] Dynamically varying the amount of transmitted power can
adversely affect the demodulation process in the mobile station
receiver. In the receiver, the optimal process is weighting the
accumulated symbol amplitude by the signal to noise ratio for each
symbol. Such a weighting process is described in U.S. Pat. No.
6,101,168 entitled "METHOD AND APPARATUS FOR TIME EFFICIENT
RETRANSMISSION USING SYMBOL ACCUMULATION," which is owned by the
assignee of the present invention and the contents of which are
hereby incorporated herein by reference. In most IS-95
implementations, the weighting uses the common pilot signal because
the code channel power is constant over a frame and the pilot
E.sub.c/l.sub.0 is a scaled value of the signal to noise ratio.
With fast forward link power control (as described in U.S. Pat.
application No. 08/842,993 cited above), the power can be varied in
a frame so that the power of a code channel is not in constant
proportion to the common pilot signal. Power variations within a
frame are not a problem because the mobile station can develop an
appropriate weighting if necessary. However, when the base station
reduces the transmitted energy of a code channel in order to use it
on one or more other code channels, the weighting can be very
different and the mobile station may not be aware of the power that
the base station is using. For example, the weighting applied to
the ABR stream 14f of the graphical representation 30 at the end of
the first frame can be much greater than that applied at the end of
the third frame. It will be understood that a large amount of power
is transmitted for the stream at the end of the first frame and
that little power is transmitted at the end of the third frame. For
an accurate weighting in such situations, the mobile station can
estimate the energy and noise in the received symbols and apply the
appropriate weighting.
[0052] Rather than using the common pilot channel for weighting as
described in the paragraph above, it is also possible to develop
the weighting using a dedicated pilot channel. A dedicated pilot
channel is a pilot that is directed to a specific mobile station.
The dedicated pilot power would be part of the power that is being
transmitted to the specific mobile station. With the dedicated
pilot, it may be possible to adjust the pilot level in proportion
to the transmitted power on the data channel. A drawback of this
approach is that it has the impact of increasing the variance of
the phase estimator, thus degrading the performance. Moreover, the
dedicated pilot channel approach for weighting may not work if
there are non-ABR services being transmitted to the mobile station,
and such non-ABR services require a high pilot level for proper
performance. In such cases, the level of the dedicated pilot will
be maintained at a high level, thereby wasting power and precluding
the use of the dedicated pilot channel for development of the
weighting.
[0053] Under the above conditions the mobile station may not
receive the ABR traffic stream with sufficient power to demodulate
the stream with few enough errors (i.e., to demodulate the stream
correctly). The mobile station can use a combination of checking
the cyclic redundancy check (CRC) bits, testing the re-encoded
symbol error rate, and checking the total received energy in order
to determine whether the frame is significantly erred. Other
techniques known by those skilled in the art can also be used.
[0054] In accordance with the present invention, when a frame is
determined to be in error, the mobile station stores the received
code symbols for the frame in a buffer. In accordance with one
embodiment of the present invention, the mobile station then
computes (E.sub.t/N.sub.0).sub.k based upon the energy received in
the frame. The amount of additional (E.sub.t/N.sub.0).sub.k
required for the frame to be demodulated with the required error
rate can then be estimated. The mobile station sends to the base
station a negative acknowledgment and may include such an estimate
of the amount of additional (E.sub.t/N.sub.0).sub.rk required. The
total required (E.sub.t/N.sub.0).sub.k can be estimated in this
power control method based upon the outer loop power required (or
the threshold) for the fundamental channel or DCCH channel. U.S.
patent appl. No. 08/842,993 (cited above) discloses a method for
estimating the total required (E.sub.t/N.sub.0).sub.k based upon
the outer loop power required. Alternately, there can be a separate
outer loop power control method for the channel being used. It will
be understood that if the frame is received incorrectly (i.e., with
an undesirable number of errors), then (E.sub.t/N.sub.0).sub.k is
insufficient. Thus, the optimum power level can be determined by
conditional statistics that take into account the fact that
previous attempts were received incorrectly. Instead of sending the
amount of additional (E.sub.t/N.sub.0).sub.rk that is required, the
mobile station can send the amount of (E.sub.t/N.sub.0).sub.rk that
was received to the base station. The mobile can also include an
estimate of the amount that it expects to need for correct
demodulation in information sent to the base station.
[0055] FIG. 5 is a graphical representation 50 showing a scheduling
time line of an acknowledgment protocol between a base station and
a mobile station of a communication system suitable for
implementation of the method of the present invention. The
acknowledgement protocol of the graphical representation 50 can be
used in a power control method as set forth above.
[0056] A preferred embodiment of the method of the graphical
representation 50 can be implemented in an IS-95 third generation
system. In the IS-95 third generation system a supplemental channel
(F-SCH) can be used for transmission of the ABR traffic streams on
the forward link. The supplemental channel is typically a scheduled
channel, though it can also be a fixed or a variable rate channel.
The F-DCCH and R-DCCH are forward and reverse control channels
respectively. When the supplemental channel (F-SCH) is used for
transmission of the ABR traffic streams on the forward link in
accordance with the present invention, the error rate of the DCCH
channels is typically lower than that of the supplemental channel
(F-SCH). In the acknowledgement protocol of graphical
representation 50, the base station transmits the schedule in
medium access control (MAC) messages 94 and 98 to the mobile
station. The schedule informs the mobile station of a number of
aspects of the transmissions, which can include, but are not
limited to, the number of frames that will be transmitted, their
transmission rates, when they will be transmitted, and their frame
numbers. In one embodiment of the invention, the MAC message 94
only provides the mobile station with the transmission rate that
will be used. With this embodiment, the mobile station continually
attempts to receive the F-SCH.
[0057] The base station indicates that two radio link protocol
(RLP) frames 102, 104 must be sent to the mobile station. RLP is
the upper layer framing protocol of the communication system. An
RLP similar to that described in TIA standard IS-707 can be used,
though many different upper layer framing protocols can be used. In
what follows, an RLP frame is assumed to map exactly to a physical
layer frame though that is not necessary as part of this invention.
The sequence numbers of the RLP frames 102, 104 are k and k+1,
respectively. The RLP frames 102, 104 are transmitted during the
physical frames i+1 and i+2, respectively. When the mobile station
correctly receives the transmission of the RLP frame k+1 (104), it
acknowledges the frame using message 112. Since the base station
does not receive an acknowledgement of the RLP frame k (102), the
base station sends a new forward link assignment in the MAC message
98 indicating that the RLP frame k is scheduled for retransmission
during the physical frame i+5 (110). The mobile station learns from
the MAC message 98 that it must combine the signal received during
frame i+5 (110) with the signal received during frame i+1(102).
After physical frame i+1 is retransmitted during the physical frame
i+5, the mobile station combines the received energy for each
symbol in the retransmitted physical frame i+5 with the received
energy of the original transmission during frame i+1 (stored in the
buffer as described above) and decodes the combined received energy
of the frames as described herein.
[0058] The mobile station acknowledges the RLP frame k during the
frame i+6 using the acknowledgement message 114. With this
acknowledgement based method, the energy deficit is not transmitted
to the base station. Moreover, in further embodiments, the energy
deficit may be sent to the base station with the acknowledgement of
the RLP frame k+2. Thus, in this embodiment, the acknowledgement
always carries the estimate of the amount of additional
(E.sub.t/N.sub.0).sub.k required from the first frame that was in
error. However, this method may not work well if the last frame in
a sequence of frames is not received correctly by the mobile
station.
[0059] When the base station determines that an acknowledgement was
not received from the mobile station and it desires to retransmit
the message, the base station determines the level at which to
transmit the message. The base station can choose a level based
upon the feedback information on the amount of required energy
needed by the mobile station. Alternatively, the base station can
estimate the amount of energy that the mobile station has already
received and use this to determine the level at which to
re-transmit. The power level chosen for retransmission will, in one
embodiment, correspond to a minimum power level needed for correct
demodulation when the symbol energy of the original message and the
retransmitted messages are combined in the receiver buffer. The
base station can form an estimate of the amount of energy that the
mobile station has already received using information from forward
power control, the transmission rate, the propagation conditions,
the amount of power already used to transmit the frame, and the
path loss. The actual information used in developing this estimate
can include these or any other parameters which are available to
the base station. Alternatively, the base station can just transmit
a fixed power (or fixed power relative to the forward power control
level) to the mobile station. This fixed power level could have
been predetermined by the base station.
[0060] Instead of the explicit method of the base station
transmitting message 98 to the mobile station to provide the
identity of a retransmitted frame, the mobile station can
alternatively implicitly determine the identity of the
retransmitted frame with a reasonably degree of accuracy from the
transmitted data. For example, the Euclidean distance can be used
to determine whether frame i+5 matches the data received in
previous frames that have not been acknowledged, such as frame i+1.
Thus, the explicit retransmission of message 98 is not required for
this invention. In this alternative embodiment, the mobile station
compares the received symbols from the current frame with symbols
from all previous frames stored in the mobile station's buffer. If
the mobile station determines that the retransmitted frame
corresponds to a frame already within the buffer, the mobile
station combines the energies for each symbol and attempts to
decode the frame.
[0061] In an alternative embodiment of the protocol shown in FIG.
5, message 94 is not required. Message 94 is used in the embodiment
described above to provide an indication to the mobile station that
frames 102 and 104 are to be transmitted. In this alternative
embodiment, the mobile station can alternatively determine
implicitly whether the current frame is a new frame or a
retransmitted frame with a reasonably degree of accuracy from the
transmitted data using the Euclidean distance analysis described
previously.
[0062] FIG. 6 is a graphical representation 60 showing a scheduling
time line of a negative acknowledgement protocol between a base
station and a mobile station suitable for implementation in the
system of the present invention. The negative acknowledgement
protocol of the graphical representation 60 can be used in a power
control method as set forth above.
[0063] In the negative acknowledgement protocol of graphical
representation 60, the base station informs the mobile station of
the RLP frames 102, 104 to be transmitted and the physical layer
frames to be transmitted by means of the MAC message 94. The base
station then sends the frames 102, 104 to the mobile station. If
the mobile station does not receive the RLP frame 102 correctly,
the mobile station sends a negative acknowledgement 116 to the base
station. The base station then sends message 98 as previously
described and the information of the frame 102 is retransmitted as
the frame 110.
[0064] One of the disadvantages of the negative acknowledgement
based protocol is that the base station is not able to take action
to retransmit frame 102 if the negative acknowledgement is not
received from the mobile station. For ABR traffic, the probability
that a frame transmitted on the forward link is in error is much
greater than the probability that the negative acknowledgement sent
on the reverse link is in error. This is because the amount of
power required to transmit a frame with many bits on the forward
link is considerably higher than the amount of power required to
transmit an acknowledgment. The negative acknowledgment protocol
can use a MAC message 98 to indicate that the frame is being
retransmitted. The MAC message 98 can be similar to that used for
the acknowledgement protocol shown in FIG. 5. The negative
acknowledgement protocols can also use an implicit method for
determining the identity of a retransmitted frame that is similar
to that described for the acknowledgment protocol shown in FIG.
5.
[0065] Several alternate embodiments of the negative
acknowledgement based protocol are possible. In one alternate
embodiment, the base station does not inform the mobile station
about the frames of the original transmission and does inform the
mobile station of time intervals wherein the frames can be sent.
The mobile station demodulates all of the physical frames. If the
mobile station correctly receives the RLP frame k +1, it transmits
a negative acknowledgement for the missing frames (which includes
the kth frame) on the R-DCCH. A disadvantage of this protocol is
that the mobile station does not know when to release memory used
to store the symbol energies from the various frames. This
disadvantage can be addressed in several ways. One way is providing
a fixed amount of memory and having the mobile station discard the
oldest received physical layer frame symbol energies when it needs
additional memory. Alternatively, the mobile station can discard
memory corresponding to a physical layer frame that was received
more than a predetermined time in the past.
[0066] A further disadvantage of this protocol is that the mobile
station may not have information about when to send a negative
acknowledgement promptly for frames that are received in error.
This disadvantage is compounded by the fact that only a few frames
may be received correctly on the first transmission. This
disadvantage can be overcome if the base station occasionally
transmits a second done message to the mobile station on the
F-DCCH. This done message informs the mobile station that the base
station has transmitted a sequence of frames, thus permitting the
mobile station to determine the frames which it should have
received. The mobile station can then send a negative acknowledge
message for the frames that it did not receive. Any done message
can be combined with any other message, such as a message that
indicates that the frames will be transmitted.
[0067] Significantly, when a frame is initially transmitted with
insufficient energy to permit correct demodulation by the intended
receiver, as described above, and then retransmitted, the
retransmission provides time diversity. As a result, the total
transmit energy of the frame (including retransmissions) is lower.
In other words, the combined symbol energy for both the initial
transmission and retransmission(s) of the frame is lower than the
energy that would have been required to transmit the frame
initially at full power (i.e., at a power level that was sufficient
on its own to permit correct demodulation by the intended
receiver). This can be determined because the required
E.sub.b/N.sub.t for a predetermined bit error rate or frame error
rate is lower when this method of retransmission is used.
[0068] Furthermore, it will be understood that the fast forward
link power control (as described in U.S. patent appl. No.
08/842,993 cited above) is less important in the case of ABR
traffic streams that utilize the retransmission approach described
above. The fast forward link power control is less important
because the retransmission approach is a form of power control. In
addition, fast forward link power control may be less important
when the retransmission approach is being employed, because fast
forward link power control attempts to maintain the E.sub.b/N.sub.t
constant at the mobile station. Thus, it may be preferable to not
use fast forward power control for ABR services.
[0069] In the case of the forward link, the base station adjusts
its transmit power to the channel when it is unable to supply
additional power for the channel from the base station. This can
occur, for example, when a VBR user or a set of VBR users, a higher
priority stream a (CBR or VBR stream), or a set of high priority
streams require more transmit power due to different path losses or
propagation conditions, or when the forward link path loss
increases between the mobile unit and the base station.
[0070] The present invention has been described above with respect
to variations in base station loading for transmitting forward link
services such as CBR and VBR streams and variations due to power
control. However, it will be understood that the invention can be
advantageously applied to other situations including transmissions
on the reverse link.
[0071] In the case of the reverse link, an important parameter is
the rise in the level of the total amount of noise over the level
of the thermal noise at a base station (referred to hereafter as
the "rise over thermal"). The rise over thermal corresponds to the
reverse link loading. A loaded system attempts to maintain the rise
over thermal near a predetermined value. If the rise over thermal
is too great the range of the cell is reduced and the reverse link
is less stable. A large rise over thermal also causes small changes
in instantaneous loading that result in large excursions in the
output power of the mobile station. However, a low rise over
thermal can indicate that the reverse link is not heavily loaded,
thus potentially wasting available capacity. It will be understood
by those skilled in the art that methods other than measuring the
rise over thermal can be used to determine the loading of the
reverse link.
[0072] ABR traffic streams can also be allocated available capacity
on the reverse link to keep the rise over thermal more constant.
The base station can control the reverse link transmission with a
form of high rate RLP control. The third generation of IS-95 has a
single power control stream that controls the pilot, the R-FCH, the
R-SCH, and the R-DCCH simultaneously. Slower signaling is used in
this IS-95 embodiment to control the power allocation between the
channels. Typically the R-SCH requires most of the transmit power
since it is carrying the high rate data stream. If all channels are
controlled by the high rate power control stream, then when the
base station requires a reduction of the power on the R-SCH in
order to control loading, the power of all channels is reduced.
This is not desirable because the pilot, the R-FCH, and the R-DCCH
can be received by the base station at a level that is too low.
[0073] A separate high rate power control channel from the base
station to the mobile station can be used for reverse link power
control on an IS-95 third generation system. The power control rate
for the reverse link can be eight hundred bits per second. While
the same rate can be used to control the R-SCH independently of the
other channels, the 800 bps rate requires more base station
transmission power than necessary. Thus, the power control rate for
the R-SCH can be somewhat lower because it does not have to be
maintained perfectly in fading conditions. Furthermore, the power
control for the R-SCH can be at an offset with respect to the main
power control stream that controls the R-SCH, the R-DCCH, and the
pilot. A signaling message or other signaling scheme can be
transmitted to the mobile station to provide this relative power
control in lieu of a power control bit stream.
[0074] In an alternative embodiment, a separate low rate power
control stream can be used to provide a correction to all mobile
stations relative to their own individual power control streams.
This can be a binary stream specifying an increase or decrease in
power for mobile stations relative to their own individual power
control streams. This can also be a three level method that can
indicate increase, decrease or do not change. Additionally, any
other known power control scheme can be used for the separate low
rate power control.
[0075] The disclosed method can also be used when a mobile station
has insufficient power to transmit all of the streams to be
transmitted to an intended receiver at a receive power level that
allows correct demodulation. In such a case, the mobile station can
reduce the transmitted power on the R-SCH to attempt to maintain
the R-FCH and R-DCCH at the desired output power level. This method
is similar to a method used on the forward link. Since the base
station will receive some power from the mobile station, the amount
of power required during the retransmission will be less.
[0076] FIG. 7 shows a graphical representation 70. The graphical
representation 70 sets forth a scheduling time line of a negative
acknowledgement protocol on a reverse link between a base station
and a mobile station of a communication system suitable for use
with the present invention. The negative acknowledgement protocol
of the graphical representation 70 can be used in a power control
method as set forth above.
[0077] Most of the timing and the acknowledgement structure of the
reverse link operates in the same manner as described with respect
to the forward link. An exception is the following. In the reverse
link, the mobile station requests permission to transmit the high
rate ABR frames 164, 168 by means of the request 176. The base
station informs the mobile station when to send ABR frames 164, 168
by means of an assignment message 152. The mobile station of the
graphical representation 70 is not required to request
retransmission of an erred frame 164. However the base station
knows that the frame 164 is in error and schedules a retransmission
when the reverse link has available capacity. Furthermore, a
negative acknowledgement message 156 transmitted by the base
station can include permission to retransmit a reverse link power
frame 172 and the slot in which it is transmitted.
[0078] The alternative embodiments previously described above with
respect to the forward link can also be applied to the reverse
link. For example, in one embodiment of the reverse link, the
mobile station is not required to request transmissions using the
MAC message 176. Furthermore, the base station is not required to
grant access to the channel using the MAC messages 152. In another
embodiment, the base station is not required to explicitly inform
the mobile station using message 176 of the frame in which to
retransmit the message.
[0079] Referring now to FIG. 8, there is a block diagram showing a
base station controller (BSC) 810 that includes a scheduler 812 for
allocating forward link power among different traffic streams in
accordance with one embodiment of the present invention. The
various policies for allocating power to the ABR transmission
streams may be implemented in software using scheduler 812.
Operation of a scheduler that may be modified to include software
for allocating power in accordance with the present invention is
disclosed in U.S. patent application No. 08/798,951 entitled "NEW
AND IMPROVED METHOD FOR FORWARD LINK RATE SCHEDULING," which is
owned by the assignee of the present invention and the contents of
which are hereby incorporated herein by reference. In the
embodiment shown in FIG. 8, BSC 800 determines the power allocation
for each of the data streams being transmitted, this power
allocation information is then transmitted to base station
transceiver systems (BTSs) 820, 822, which in turn transmit the
various data streams to one or more mobile stations 830 in
accordance with the power allocation determinations made at
scheduler 810.
[0080] Referring now to FIG. 9, there is a block diagram 90 showing
two base station transceivers 820a, 822a that each includes a power
manager 821 for allocating forward link power among different
traffic streams in accordance with an alternative embodiment of the
present invention. The embodiment shown in FIG. 9 is useful in
cases where fast forward power control is being applied, because in
this embodiment the power allocation determinations are made at the
BTS's (rather than at the BSC 800), thereby eliminating the delay
resulting from transmission of the powers being transmitted on the
forward link from the BTSs to the BSC and the power allocation
information from BSC 800 to the BTSs. In the embodiment shown in
FIG. 9, the various policies for allocating power to the ABR
transmission streams may be implemented in software using power
managers 821. Each power manager 821 determines the power
allocation for each of the data streams being transmitting by the
corresponding BTS, and the BTS then transmits the various data
streams to one or more mobile stations 830 in accordance with the
power allocation determinations made by power manager 821. In
another embodiment, the scheduler 810 in the BSC can set some
general power allocation policy that the power managers 821 in the
BTSs carry out. This has the advantage that the power managers 821
can handle short term fluctuations without encountering the delay
between the BTS and the BSC and provides a consistent scheduling
policy over all data streams.
[0081] In summary, different scheduling policies are possible
during the transmissions of time frames 18a-f. A frame scheduling
policy is a set of rules for determining which of a plurality of
signals waiting to be transmitted are actually inserted into a
frame. In one scheduling policy, a base station can transmit the
traffic streams that are likely to be received at sufficient power
by the intended receiving mobile station. Alternately, a scheduling
policy can be used wherein the forward link is transmitted with
sufficient power for correct demodulation by the intended receiving
mobile station on the first transmission. In an alternate
embodiment, the base station can allocate power to a number of
different streams such that none of the streams are transmitted
with enough power to allow reliable decoding by the receiver
without at least one retransmission, as previously described. The
transmission rate and the code rate of the transmitted stream are
among the other parameters that the base station can adjust in this
case. Furthermore, one embodiment of the invention is directed to
the case wherein a mobile station has insufficient power to
transmit all of the bit streams. In this case, the mobile station
can reduce the transmitted power on the R-SCH in an attempt to
maintain the R-FCH and R-DCCH at the required power level. This
method is similar to the one used for the forward link. Since the
base station receives some power from the mobile station, the
amount of power required during the retransmission is less. It will
be understood that all of the methods disclosed herein can be used
at the time of call set up or at any time during a transmission
after set up.
[0082] The previous description of the preferred embodiments is
provided to enable a person skilled in the art to make or use the
present invention. The various modifications to these embodiments
will be readily apparent to those skilled in the art, and the
generic principles defined herein can be applied to other
embodiments without the use of the inventive faculty. Thus, the
present invention is not intended to be limited to the embodiments
shown herein but is to be accorded the widest scope consistent with
the principles and novel features disclosed. It should be further
noted that the paragraphs and subparagraphs within the claims are
identified with letter and number designations. These designations
do not indicate the order of importance of the associated
limitations or the sequential order in which steps should be
performed.
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